Every commutator we supply is described by its segment count and seven dimensions, A to G. Measure those and we can match a standard size or make one to fit. This takes about ten minutes with a digital caliper, or a bit longer with a steel rule.
What you need
- A digital caliper (a £15 one is fine) or a steel rule marked in millimetres
- A marker pen
- Your phone, for photos
- The armature out of the starter or alternator, with the commutator still on it if possible
Step 1: Count the segments
The segments are the copper bars running along the brush surface, separated by thin slots of mica. Mark one bar with the pen, then count round until you come back to it. Count twice. On a worn commutator the slots can be hard to see; a photo with the phone torch on usually brings them out.
Step 2: Measure A to G
| Letter | What it is | How to measure it |
|---|---|---|
| A | Riser diameter | Across the riser, the raised ring of hooks or slots the windings are soldered into. Measure hook tip to hook tip on opposite sides. |
| B | Bore | The hole the shaft passes through. Measure the inside diameter at the end of the commutator, not the shaft itself, as commutators are often pressed on with an interference fit. |
| C | Brush diameter | Across the copper brush surface. On a worn part measure an unworn edge, or add the depth of the wear groove back on. This is the dimension that shrinks in service. |
| D | Total length | End to end, including the riser. Do not include any spacer or washer that sits against it. |
| E | Riser width | The thickness of the riser ring along the shaft axis. |
| F | Riser slot width | The width of one slot in the riser, where a winding lead sits. Measure across the slot opening with the caliper jaws or a feeler gauge. |
| G | Riser slot depth | How deep the slot goes into the riser, measured from the outside of the riser to the bottom of the slot. The caliper's depth rod is the easiest way. |
Write them down in millimetres to one decimal place. If a measurement is awkward, say so when you send it rather than guessing; we would rather check than make the wrong part.
Step 3: Note the type
Most starter commutators are groove (hook) type: the windings hook into slots on the riser and are soldered or welded. Some older and larger units use a riser type with tall copper risers, and small alternator and motor units can be flat (face) commutators where the brushes run on the end. A side-on photo tells us which you have.
Step 4: Find the OEM number
Starter armatures often carry a stamped or etched number on the shaft or the lamination stack, and the starter body carries the maker's part number (Bosch numbers look like 0 001 123 456 or 1 004 007 014). Either one lets us cross-reference without any measuring at all.
Step 5: Photograph it
- One photo square-on to the end, so the segments can be counted
- One from the side showing the riser and the hooks
- Daylight or a bright lamp, plain background, in focus. A caliper in shot showing a reading is a bonus
Send it over
Put the numbers and photos into the made-to-measure form, or email them to sales@ukcommutators.co.uk. We reply to every enquiry within one working day.